A copolymer of tetrafluoroethylene (hereinafter referred to as TFE) and hexafluoropropylene (hereinafter referred to as HFP) (hereinafter referred to as FEP) has heat resistance, chemical resistance, electric insulating properties, non-adhesiveness and low frictional properties equal to those of polytetrafluoroethylene (hereinafter referred to as PTFE) and therefore is often applied not only in the chemical industry, electric and electronic industries and machine industry, but also in a wide range of fields, from space development and aircraft industry to household products. Furthermore, FEP has melt flowability and can be melt processed by compression molding, extrusion molding, injection molding and fluid immersion coating.
However, in comparison to general thermoplastic resin, FEP has an extremely high melt flow temperature and high melt viscosity and therefore, a molding temperature of 330° to 420° C., in some cases 440° C., is required. Molding at such high temperatures causes thermal decomposition of the polymer. Furthermore, when extrusion pressure is raised in order to increase the molding speed, surface roughness of the extruded article and abnormal flow (melt fracture) are caused.
In order to solve the above problems and improve moldability without losing properties, JP-A-52-98761 discloses a fluororesin composition, in which PTFE is compounded in FEP. As a result, phenomena such as surface roughness of the molded article and melt fracture are said to have difficulty occurring, but the effects are still insufficient.
Also, in recent years, FEP is used for cable coating material and the speed for molding may reach at least 1200 ft/minute and in some cases, 3000 ft/minute. This speed is within the melt fracture range of FEP. Consequently, the coated cable is large in wire diameter unevenness and electric properties such as insulation become uneven. Furthermore, the phenomenon of cone break occurs, that is the conular part formed in the die when coating comes off, and coating at high speed is difficult.
Furthermore, in order to improve electric properties, an electric wire can be coated with resin in a foamed state. However, to obtain stable electric properties, the resin must be in a foamed state of even bubbles with a small bubble diameter.